1. Field of the Invention
The present invention relates to a magnetic separator (namely, a magnetic separation apparatus) used for coal cleaning and to a pulverized coal combustion (or firing or burning) apparatus using the magnetic separation apparatus (or system).
2. Description of the Related Art
FIG. 9 is a diagram illustrating the principle of magnetic separation, which shows the distribution of lines of magnetic force of quadrupole magnets and the magnetic field strength distribution in an inner duct thereof. In this figure, reference numeral 40 designates quadrupole magnets that have a quadrupole structure provided in a circle therein. The lines 41 of magnetic force are generated by this magnet 40. Here, let (dH/dZ), H, X and V denote magnetic field gradient, magnetic field strength, magnetic susceptibility and the volume of a fine particle, respectively. Magnetic force Fm given by the following equation (1) acts in the direction of the magnetic field gradient: EQU Fm=.chi..mu..sub.o VH(dH/dZ) . . . (1)
Incidentally, in the aforementioned equation (1), .mu..sub.o denotes an absolute permeamibility of vacuum. Magnetic separation is performed by utilizing a variation in this magnetic force, which is caused due to a change in the magnetic susceptibility of this fine particle. As illustrated in FIG. 9, the magnetic force Fm given by the aforementioned equation (1) acts upon a fine particle contained in the inner duct 42 of the four-pole magnet 40. Variation in this magnetic force results in a change in the position in the Z-direction of each fine particle, so that the magnetic separation is achieved.
FIG. 10 is a diagram conceptually illustrating a section of a conventional magnetic separator employing such a principle of magnetic separation and illustrates an apparatus 110 for separating a dry paramagnetic material from a dry diamagnetic fine-grain material. Hereinafter, this apparatus will be described in detail.
A wall 114 and an inner space 116 vertically extend in the axial direction of the apparatus and thus compose a cylinder 112. Rotary screw 118 is installed in the cylinder 112. The screw 118 consists of the shaft 120 and a helical blade 122. The helical blade 122 is angled downwardly in both of the radial and axial directions and is accommodated inside the wall 114. The screw 118 is connected to a motor 124 and is rotated by this motor 124, so that fine particles are carried from the top portion of the screw 118 downwardly. A vibration drive or exciter (namely, a shaker) 126 is connected to the screw 118. The screw 118 is shaken during the rotation thereof.
A magnet 128 is disposed around the wall of the cylinder 112. This magnet 128 acts the magnetic field in the inner space 116. Thus, there is constructed a four-pole magnet, by which the magnetic field gradient is given in the inner space 116. The magnetic field (strength) has a maximum value on the wall 114. Further, the magnetic field strength at a point decreases as the point approaches the shaft 120. This magnetic field is constant in a central zone 129. In an edge portion 132, the magnetic field decreases linearly in the upward direction from an edge 132 of the magnet 128.
To enhance the separation ability or performance, a pulverizer 134 is used. Pulverized coal is sent by an auger 136 to a movable coal feeder 138. Then, the pulverized coal is sent from the coal feeder 138 to the helical blade 122. Rotation of the screw 18 brings the pulverized coal into the apparatus 110.
Paramagnetic fine particles of kaolinite and ash undergo the magnetic force Fm illustrated in FIG. 9 and move toward the wall 14, while fine particles of combustible (organic) content contained in the coal, which are diamagnetic fine particles, move along the direction of the shaft 120.
A splitter 142 is provided in a lower portion of the apparatus 110 and is composed of three concentric cylinders or tubes 144, 146 and 148. The tube 144 collects fine particles (incidentally, major constituents thereof are diamagnetic fine particles), which approaches the shaft 120. The tube 148 collects fine particles (incidentally, major constituents thereof are paramagnetic fine particles), which approaches the wall 114. The tube 146 collects a mixture of antimagnetic and paramagnetic fine particles.
The aforementioned conventional magnetic separator 110 has the following problems:
(1) Mechanical drive parts such as the screw 118, the rotary shaft 120, the motor 124, the vibration exciter 126 and the helical blade 122 are provided in the inner duct of the quadrupole magnet, so that the clogging thereof owing to the fine particles and the abrasion thereof occur. PA1 (2) Residual magnetization occurs in machine parts with the result that the fine particles adhere to the blade and so forth. Consequently, the separation performance is degraded. PA1 (3) Leakage current occurring in the rotating parts results in the generation of heat and in the occurrence of rotation loss. PA1 (4) When using a superconducting magnet in the case that electric current is directly supplied to the quadrupole magnets from the power supply, losses occur in an electric lead and in a normal conducting part of the power supply. Further, in the case of using the normal conducting magnet, losses are produced in the entire system. PA1 (5) In the case that the aforementioned conventional magnetic separator 110 is applied top as an apparatus of separating combustible (organic) materials from incombustible materials (such as pearlite, kaolinite and ash), if this magnetic separator is provided separately from a combustion apparatus, there is the necessity of labor and space for the storage, retention and conveyance of separated materials. Consequently, the cost of the magnetic separator is increased. PA1 (1) Namely, there is provided a magnetic separator (hereunder sometimes referred to as a first magnetic separator of the present invention) which comprises a cylindrical inner duct disposed along the vertical direction; quadrupole magnets constituted by superconducting coils, which are placed around the aforesaid inner duct and are cooled by liquid helium and are excited by use of a DC power supply; fine-particle supply means for ejecting and dropping fine particles, which consist of a plurality of elements and compounds, from a top end of the aforesaid inner duct; and first and second collection tubes, which are provided at a bottom portion of the aforesaid inner duct, for separating and collecting a paramagnetic material and a non-magnetic material, which are included in the aforementioned fine particles, owing to a difference between magnetic forces respectively acted on the aforementioned fine particles, which are ejected from the aforementioned fine particle supply means and drop. PA1 (2) Further, in the case of an embodiment (hereunder sometimes referred to as a second magnetic separator of the present invention) of the magnetic separator described in the aforesaid first magnetic separator of the present invention, the aforesaid fine-particle supply means comprises an umbrella-like baffle plate disposed in the aforesaid top end portion, wherein fine particles are ejected upwardly toward the aforesaid baffle plate from a lower portion thereof, and then the fine particles having collided with the aforesaid baffle plate are caused to drop toward the aforesaid inner duct. PA1 (3) Further, in the case of an embodiment (hereunder sometimes referred to as a third magnetic separator of the present invention) of the magnetic separator described in the aforesaid first or second magnetic separator of the present invention), the aforesaid quadrupole magnets comprise normal conducting coils and are cooled with liquid helium. PA1 (4) Further, in the case of an embodiment (hereunder sometimes referred to as a fourth magnetic separator of the present invention) of the magnetic separator described in the aforesaid first or third magnetic separator of the present invention), a persistent current circuit switch is connected in parallel with the aforesaid quadrupole magnets. PA1 (5) Further, in the case of an embodiment (hereunder sometimes referred to as a fifth magnetic separator of the present invention) of the magnetic separator described in the aforesaid first or second magnetic separator of the present invention), an intermediate-material collection tube, which is operative to collect intermediate materials, and a bypass collection tube, which is operative to supply the material collected by the aforesaid intermediate- material collection tube, are provided between the aforesaid first and second collection tubes. PA1 (6) Furthermore, there is provided a pulverized coal combustion apparatus (hereunder sometimes referred to as a sixth apparatus of the present invention) that comprises: